fix error without feature flag in LegacyGeo (#81494)

modules/legacy-geo/build.gradle

@@ -6,6 +6,8 @@
  * Side Public License, v 1.
  */
 
+import org.elasticsearch.gradle.internal.info.BuildParams
+
 apply plugin: 'elasticsearch.internal-cluster-test'
 apply plugin: 'nebula.optional-base'
 
@@ -21,6 +23,12 @@ dependencies {
   testImplementation project(":test:framework")
 }
 
+if (BuildParams.isSnapshotBuild() == false) {
+  tasks.named("test").configure {
+    systemProperty 'es.index_mode_feature_flag_registered', 'true'
+  }
+}
+
 tasks.named("thirdPartyAudit").configure {
   ignoreMissingClasses(